Instar and Life Stages of Triatomines

Using Rhodniusprolixus as an example, the sex life of female triatomines begins one to three days after the bug’s emergence into the adult stage (Molyneux and Ashford 1983:81). Males develop a bit more slowly and inseminate females five to nine days after their emergence as adults. As mentioned, wings are sprouted and used only at the adult stage, primarily for copulation and reproduction purposes. Flying necessitates large amounts of energy. The male flies on top of the female to embrace her with his legs, and then slides alongside, inserting his reproductive organ into her. Females are inseminated shortly after molting and can produce viable eggs for one year after separation from males (Perlowagora‑Szumlewiecz 1969). Multiple copulation does not affect egg production. Although blood meals are necessary for viable egg production, unfed mothers may lay fertile eggs if they maintained a high nutritional status as nymphs.

Within ten to twenty days, the female deposits her eggs. In laboratory conditions, the eggs of T. infestans are clearly visibleivory colored and about half the size of a grain of rice. Laboratory‑fed females lay anywhere from 80 to 150 eggs; in natural environments the crucial variable for egg production is the availability of blood meals. Egg production also varies among species, with certain species producing up to 2,000 eggs (Molyneux and Ashford 1983:80). Triatoma infestans adults live from eight to sixteen months and lay an average of 240 eggs (Zeledón and Rabinovich 1981). The life span of T. infestans in Bolivia has been estimated at three years (Jemio Alarico and Ariel Sempertegui, interview 6/18/1991).

Females lay their eggs in their microhabitat, in such places as cracks or crevices in the homes, burrows, or nests of the mammalian or avian hosts of the bugs. Some eggs are laid loose and others are glued in clumps to a surface. The eggs hatch in from one to two weeks, depending on the temperature. The emergent triatomine will pass through five nymphal instars on its way to growing wings and becoming an adult. An instar is the period between the nymph stages in the life of an insect. During the nymph stages, triatomines crawl. The nymph stages can take anywhere from four to forty‑eight months, depending on the temperature, humidity, and frequency and volume of blood meals. Nymphs can imbibe between six to twelve times their own weight in blood.

Triatomines need an adequate blood meal for growth and development during each of the five immature stages and for oviposition by the adult females (Marsden 1983:266). The content of blood meals increases with each nymph stage, adult bugs drinking about 100‑300 mg of blood, if possible. Among certain species, fifth‑instar nymphs can consume more blood than adults, and female adults more than male adults (Molyneux and Ashford 1983:81). Instars from the third to fifth stage are frequently used in xenodiagnosis. At all stages they take many times their weight in blood, and the molt from a fifth‑stage larva to an adult is particularly dependent on a large blood meal.

Although triatomines can go for months without blood meals, many take their meals after seven days, which provides sufficient time for T. cruzi to have multiplied in the bug with its six‑to‑fifteen‑day reproduction cycle. When their hindgut becomes extended with feces, triatomines seek out blood meals, which is a distinct advantage for the parasitic T. cruzi.

Triatomines can live a long time without blood; T. infestans is able to tolerate longer periods of starvation throughout the later instars. During the fourth and fifth instars the organism can survive up to seven months without food. Younger nymphs need blood meals more frequently in order to pass through the stages to adulthood. Long periods of starvation don’t affect the persistence or viability of trypanosomes in the insect’s gut (Molyneux and Ashford 1983:80). T. cruzi have no known pathological effects on vinchucas, and to what degree this parasitic relationship negatively affects a triatomine’s metabolism is not known. Conversely, T. cruzi have adapted to the gut of vinchucas, an environment equally as hostile as that of their human hosts.

Of epidemiological significance, vinchucas are capable of being vectors for T. cruzi throughout all their nymph stages. However, they have to become initially infected from ingestion of the parasite in a blood meal from a host; they are not infected from birth. It follows that there will be lower degrees of infection among early nymph stages, because some have not taken as many blood meals. Significantly, the proportion of infected bugs rises with age, and the adult infection rate can be used as a convenient measure of transmission risk in the field (Minter 1978b; Marsden et al. 1982).

In Tarija, Bolivia, for example, vinchucas within the nymph stages had an infection rate of 25 percent, whereas within the adult stage it was 37 percent (Valencia 1990a). The infection rate of vectors, however, greatly depends on the degree of infection found in hosts. In certain regions of Bolivia, as high as 90 percent of adult vinchucas are infected, as are 50 to 60 percent of the nymphs. Thus, variables affecting the rate of bugs infected with T. cruzi are its developmental stage, access to blood meals, and the incidence of infected hosts. In a household colonized with vinchucas which has one person with Chagas’ disease, it is only a matter of time until all members will become infected.

Although the feeding activities of domestic vectors are not well known, Rabinovitch et al. (1979) have calculated that within a house with a typical insect population of Rhodnius prolixus, an adult would be bitten an average of nine times each night and in exceptional circumstances up to fifty‑eight times. Bolivians can take some encouragement from the fact that Rhodniusprolixus is much more voracious than Triatoma infestans, whose populations average only two bites per adult per night. The least bothersome is Panstrongylus megistus; Marsden (1983:258) sat for two nights in a house with many P. megistus present on the walls and was approached by only a single bug. Some bugs remained on the same spot of the wall for as long as forty‑eight hours.

Triatomines are biologically equipped to find blood meals. Heat‑sensitive antennae and pheromonal communication direct them to sleeping mammals and birds at night. Frequently, they glide back and forth across a room, detect warmth from a human and gently land upon his or her body or face. Probing the warm surface to sample underlying fluids, sensilla in the insect’s proboscis search for adenosine triphosphate (ATP) or other appropriate engorgement factors. If successful, the vinchuca injects its needle‑sharp proboscis coated with a general anesthetic and decoagulant so that it can easily and leisurely (up to thirty minutes) drink blood, as much as seven times the bug’s weight, until its gut is filled to a certain capacity determined by abdominal stretch, which triggers the molting cycle in nymphs (Molyneux and Ashford 1983:79). Abdominal stretch of the midgut with blood also triggers peristalsis and the triatomine defecates on its victim’s face, adding insult to injury (as well as exchange of parasites for blood).

Genetic factors of different triatomine species regulate their susceptibility to parasites, parasite density in the feces, and defecation time (WHO 1991). Defecation time of triatomines is crucial for the transmission of parasites to hosts. Scratching the wound, the victim spreads the fecal matter over the bite site, allowing parasites to enter through the wound or the irritated surrounding skin.

After filling up with blood, T. infestans defecates within minutes near the bite site. Other triatomines defecate some time after the blood meal and away from the bite site, which lessens chances of spreading infection (Wood 1951). Studies of three species have also shown important differences not only between species but also between the different instars and sexes within the species (Dias 1956, Zeledón et al. 1975, Pippin 1970). After a blood meal, R. prolixus was the first to defecate; it also defecates more frequently, within one‑, five‑, or ten‑minute intervals, than other species. The mean times for the first defecation and the percentage of individuals which defecated within this species was surpassed only by the adult stages of T. infestans. T. dimidiata was slower to defecate and did so less frequently after meals than the other two species. However, T. dimidiata defecated more during the meal, taking longer to feed and interrupting the blood meal more frequently to defecate. Females of all three species defecated sooner, more frequently, and in greater number than did males. T. dimidiata females and fifth‑instar nymphs had the highest defecation average. T. infestans defecated throughout all instar stages but mostly so during the fourth instar stage and less so as an adult. R. prolixus also defecated throughout all instar stages, with the males defecating decisively less.

A defecation index has been established by Zeledón (1983:329) for each of the instars, which permits an easy visualization of the differences between instars of the same species or of different species. The index is the mean number of defecations in ten minutes, multiplied by the percentage of insects which defecate in ten minutes divided by 100. The high defecation index of T. infestans and R. prolixus is a major reason why these triatomines are so successful in transmitting T. cruzi.

Nonetheless, defecation at later times also presents health hazards in that fecal matter is dragged across bedding, floors, walls, and other parts of the room, where it presents occasions for contamination. T. cruzi remain alive in fecal matter as long as it stays moist, and this depends on the humidity and the amounts of fecal matter. If the fecal matter falls from the ceiling into liquids or food, parasites can survive for long periods and can contaminate a large number of people at one time. Already discussed, schoolchildren in Brazil were infected with Chagas’ disease from contaminated milk (Mazza 1936).